To show that more than one person share the same item together, make only the last owner in the series possessive.
Examples: Ken and Larry's ice cream
(They share the same ice cream.)
John and Mary's pet cats
(They share the same cats.)
To show that there are similar items which are owned individually by different owners, make each owner in the series possessive.
Example: John's and Mary's pet cats.
(They each have their own pet cat or cats.)